Method and device for monitoring the wear condition of a tool

ABSTRACT

In a method for monitoring the wear condition of a tool ( 2 ) during machining of a metal workpiece ( 4 ) by means of a machine tool ( 1 ) by detecting measuring signals generated during the machining process with the aid of at least one sensor ( 6 ) and by transforming said measuring signals detected by said sensor ( 6 ) into wavelet coefficients by means of a wavelet function, wherein the resultant wavelet coefficients are used for calculating wear condition parameters for the tool ( 2 ), it is provided that the vibrations, which are generated during the machining process, of a vibration system comprising at least the tool ( 2 ) and the metal workpiece ( 4 ) are detected by at least one vibration sensor ( 6 ).

BACKGROUND OF THE INVENTION

[0001] The invention relates to a method and a device for monitoring the wear condition of a tool during machining of a metal workpiece by means of a machine tool.

[0002] Said wear monitoring process aims at deducing the wear progress of the tool from the sensor signal detected during the machining process. For this purpose sensors are installed in the working space of the machine tools such that the wear condition of the tool can be deduced from the detected sensor signal.

[0003] From JP 9323239 A a wear detector for lathe tools and a monitoring process are known, wherein the torsion moment acting upon the tool is measured and the measuring signals are supplied to a wavelet converter. By transforming the measuring signals from the sensor into wavelet coefficients with the aid of a wavelet function parameters can be determined which are representative of the wear condition.

SUMMARY OF THE INVENTION

[0004] It is an object of the present invention to improve the aforementioned process and the aforementioned device such that a quantitative and qualitative analysis of the wear condition with improved reproducibility is possible.

[0005] The present invention advantageously provides for reception by at least one vibration sensor of the vibrations, which are generated during the machining process, of a vibration system comprising at least the tool and the metal workpiece. The analysis of the vibration signal is more sensitive by approximately a factor of 50 with regard to the wear condition of the tool thus allowing a statement, which is exacter in terms of quantity and quality, on the wear condition of the tool. The determined wear condition parameters allow an evaluation of the wear progress at differerent locations of the tool. This is particularly important in the case of difficult-to-machine workpieces where it is essential that the workpiece is machined without any exchange of tool taking place. In this manner it can be decided prior to commencement of machining of a new workpiece whether a tool needs to be exchanged. The vibration analysis is superior to a large extent to the measurement of a torque or force signal since the changes in the force or torque signals do not necessarily allow conclusions to be drawn with regard of the wear condition, and said changes may be due to other reasons.

[0006] The vibration signal may be detected in the three-dimensional space in at least one space axis.

[0007] According to a partcularly preferrred aspect the vibration signal is detected in all three space axes. The wear condition parameters detected along the three space axes allow detection of the wear locations along the area of contact between the tool and the workpiece and/or along the cutting edge of the tool. In this manner a qualitative evaluation of the wear condition is possible and it can, e. g., be determined whether the tool is still usable for certain applications.

[0008] The three-dimensional vibration signal can be individually evaluated in each space axis, which results in a respective direction-oriented vibration signal. By means of the wavelet function respective direction-oriented wear condition parameters can be determined from the respective wavelet coefficient.

[0009] The three-dimensional vibration signal can be individually evaluated in each space axis, wherein after transformation of the three direction-oriented vibration signals a single resultant wear condition parameter can be determined from the wavelet coefficients in each space axis by means of the wavelet function, e. g. by addition, ratio formation, multiplication or vector addition.

[0010] According to another alternative the three-dimensional vibration signal can be individually evaluated in each space axis, wherein after transformation of the three direction-oriented vibration signals three wear condition parameters are determined from the respective wavelet coefficient by means of the wavelet function, the three wear condition parameters being combined, e. g. by addition, ratio formation or multiplication, to form a single wear condition parameter.

[0011] Alternatively, the vibration signal can be detected on the workpiece, the workpiece receiving means or the machine tool components directly or indirectly supporting the workpiece receiving means.

[0012] An acceleration sensor, a vibration velocity sensor, a vibration path sensor or, in the higher-frequency range, a solid-borne sound sensor or a sound emission sensor may be used as a vibration sensor.

[0013] Preferably, the wear condition of a workpiece is monitored during machining with interrupted cut.

[0014] A particularly preferred application is the wear condition monitoring of a milling tool during free-form milling.

[0015] The monitoring process can be e. g. applied during finish milling using a spherical milling tool. Here, the tool is exposed to different contact conditions due to permanently varying contact surfaces.

[0016] In the case of repeated machining actions on similar workpieces the wear condition can be evaluated on the basis of the change in the wear condition parameter relative to an initial parameter or relative to a given limit parameter at comparable cutting, engagement and/or contact conditions between tool and workpiece on similar or different workpieces.

[0017] In the field of tool manufacture frequently only a single formed part is produced such that it is necessary to define the given measuring locations with comparable cutting, engagement and/or contact conditions on one and the same workpiece.

[0018] The given measuring locations with comparable cutting, engagement and/or contact conditions between tool and workpiece can be determined from data contained in an NC production program of the machine tool, for which purpose e. g. the path and position data can be used. These measuring locations can be determined by a software which analyzes the NC production program in a computer (postprocessor). Said software may e. g. supplement the NC production program by program steps at the determined comparable measuring locations, said program steps allowing identification of the comparable measuring locations. Thus the NC production program can control an evaluation unit which compares and evaluates the wear parameters at the given measuring locations.

[0019] The software for analyzing the NC production program may further be capable of classifying a plurality of different comparable measuring locations, said measuring locations comprising different contact zones between tool and workpiece along the cutting edge of the tool but nevertheless comprising similar or comparable cutting and engagement conditions.

[0020] The given measuring locations with comparable cutting, engagement and/or contact conditions between tool and workpiece can be determined as a function of one or a plurality of the following parameters, namely workpiece blank geometry, workpiece finished part geometry, intermediate cut surfaces, machining parameters and tool geometry.

[0021] According to a preferred aspect of the present invention it is provided that, as a function of the wear condition parameters, the wear condition of the tool is indicated in terms of quantity on a two-dimensional or three-dimensional representation of the tool along the cutting edge of the tool. The representation of the tool can e. g. be shown on a display unit of the tool machine and the wear zones can be marked on the tool contour. On the basis of the optically displayed qualitative and quantitative wear condition an operator can decide in time whether the tool needs to be exchanged.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Hereunder an embodiment of the present invention is explained in detail with reference to the drawings in which:

[0023]FIG. 1 shows a representation of a comparison of a fit-for-use tool with a worn tool,

[0024]FIG. 2 shows the cutting, engagement and contact conditions between tool and workpiece and their influencing variables, and

[0025]FIG. 3 shows a schematic representation of the determination of comparable measuring locations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The schematic representation next to the graphical representation of FIG. 1 shows the machining of a workpiece 4 using a tool 2, preferably a milling tool of a tool machine 1. For the sake of clarity only the housing of the machine spindle of the tool machine 1 is shown, a vibration sensor 6 being fastened to said housing, said vibration sensor 6 detecting the vibration signals produced during the machining process. The detected vibration signals are emitted by a vibration system comprising at least the tool 2 and the metal workpiece 4. The vibration sensor 6 may also be fastened to another location of the tool machine, wherein the place of fastening should be as near to the contact location between tool 2 and workpiece 4 so that the vibration signal is not excessively attenuated. Particularly preferred is the use of a three-dimensional vibration sensor 6 which allows direction-oriented detection of the vibration signal.

[0027] The time-related signal patterns detected by the vibration sensor 6 measure the vibrations caused by the excitations generated during the machining process. To emphasize signal characteristics which change to a particularly large extent with the wear of the tool, the signal is transformed by means of the wavelet analysis. The wavelet analysis is published e. g. in: M. Holschneider, R. Kronland-Martinet, J. Morlet, Ph. Tchamitchian: “A Real Time Algorithm For Signal Analysis with the Help of Wavelet-Transform”, in Wavelets, Time-Frequency Methods and Phase Space, J. M. Combes, A. Grossmann and Ph. Tchamitchian, Eds. Berlin: Springer, ITPI 1989, pages 286 to 297.

[0028] For example, the wavelet transformation can be performed according to the Daubechies method (Daubechies: The wavelet transform, time-frequency localization and signal analysis. IEEE Transactions on Information Theory 36(5) (1990), pages 961 to 1005).

[0029]FIG. 1 shows a comparison of the measuring signal of a fit-for-use tool with that of a worn tool 2. Said measuring signals are subjected to a wavelet transformation. The wavelet coefficients of the vibration signal are the result of the wavelet transformation, the level and distribution of the coefficients of fit-for-use tools and tools displaying different degrees of wear clearly differing from each other. The resultant patterns of the wavelet coefficients are compressed to form wear parameters or used as a basis for calculating the wear parameters which can be employed for evaluating the wear condition of the tool. If the wear condition parameters exceed defined limit values (indicated as wear limit value in the drawing), the tool is to be considered a worn tool.

[0030] From the representation of FIG. 1 showing the wavelet coefficients of the vibration signal for comparison purposes it can be seen that the coefficient of a worn tool 2 rises to considerably higher values than that of a fit-for-use tool. The representations show the respective coefficient value as a function of the coefficient number. From the wavelet coefficient the wear condition parameters can be calculated. The lower representation of FIG. 1 shows the dependence of the wear condition parameters on the wear mark width. The representation further contains a wear limit value, and when this limit value is reached, the tool 2 should be exchanged.

[0031] When free formed surfaces are milled, permanently changing cutting, engagement and/or contact conditions between tool 2 and workpiece 4 occur during the machining process due to the complex geometry of the machined workpieces.

[0032] The influencing variables shown in FIG. 2 lead to different contact conditions and changing chip-formation mechanisms. Above all, the variation of geometrical parameters, e. g. the orientation between tool axis and workpiece surface, influence to a large extent the paths of movement of the individual cutting edge points through the material volume to be removed. Depending thereon changing patterns of the width and thickness of cut occur during engagement of the cutting edge thus leading to changed dynamic excitations generated during the machining process, the effects of which are detected by the vibration sensor 6.

[0033] In FIG. 2 the variables influencing the cutting, engagement and/or contact conditions during the milling of freely formed surfaces are shown.

[0034] Important influencing variables are the machining parameters, the milling strategy, the tool geometry and the local surface topography.

[0035] For consideration of the changing cutting, engagement and contact conditions

[0036] during wear condition evaluation the analysis of the wear condition is performed at defined measuring locations of the workpiece 4. The mesuring location analysis aims at carrying out the wear analysis at a plurality of different contact zones along the cutting edge of the tool at similar or comparable cutting, engagement and contact conditions in one or different workpieces 4. For this purpose information on the geometry of the workpiece 4 and the tool 2 as well as their orientation relative to each other is required, said information being available in an NC production program of the corresponding machine tool 1. The cutting, engagement and contact conditions occurring during the machining process can be combined in classes of similar or comparable machining situations as is shown by way of example in FIG. 3. On the basis of the NC program data and the description of these defined measuring-location classes suitable contact zones along the cutting edge of the tool are determined and used for wear condition evaluation purposes. At the measuring locations belonging to a measuring-location class the wear condition parameters are analyzed by comparison such that the wear progress can be tracked and detected during the machining process.

[0037] The determined wear condition parameters can be indicated in terms of quantity by means of a two-dimensional or three-dimensional representation of the tool on a display unit, whereby the quantitative and the qualitative wear condition of the tool is immediately recognizable for the operator of the machine tool 1.

[0038] The quantity of wear can e. g. be shown by different colours on the display. By defining wear condition limit values or limit values for the rise of the measuring signals excessively worn contact zones may be e. g. marked in red. The operator can thus see whether the tool is still usable for certain production programs.

[0039] Although preferred embodiments of the invention have been specifically illustrated and described herein, it is to be understood that minor variations may be made without departing from the spirit and scope of the invention, as defined in the appended claims. 

What is claimed is:
 1. Method for monitoring the wear condition of a tool (2) during machining of a metal workpiece (4) by means of a machine tool (1) by detecting measuring signals generated during the machining process with the aid of at least one sensor (6) and by transforming said measuring signals detected by said sensor (6) into wavelet coefficients by means of a wavelet function, the resultant wavelet coefficients being used for calculating wear condition parameters for the tool (2), wherein the vibrations, which are generated during the machining process, of a vibration system comprising at least the tool (2) and the metal workpiece (4) are detected by at least one vibration sensor (6), and in the case of varying cutting, engagement and/or contact conditions between tool (2) and workpiece (4) the wear condition is evaluated on the basis of changes in the wear condition parameter at given measuring locations with comparable cutting, engagement and/or contact conditions on similar or different workpieces (4).
 2. Method according to claim 1, wherein the vibration signal is detected in the three-dimensional space in at least on space axis.
 3. Method according to claim 1, wherein the vibration signal is detected in the three-dimensional space in all space axes.
 4. Method according to claim 3, wherein the three-dimensional vibration signal is individually evaluated in each space axis and results in a direction-oriented vibration signal such that by means of the wavelet function respective wear condition parameters are defined from the respective wavelet coefficients.
 5. Method according to claim 3, wherein the three-dimensional vibration signal is individually evaluated in each space axis, and after transformation of the three direction-oriented vibration signals wavelet coefficients forming a wear condition parameter are determined from the respective wavelet coefficients by means of the wavelet function, e. g. by addition, ratio formation, multiplication or vector addition.
 6. Method according to claim 3, wherein the three-dimensional vibration signal is individually evaluated in each space axis, and after transformation of the three direction-oriented vibration signals three wear condition parameters are determined from the respective wavelet coefficients by means of the wavelet function, the three wear condition parameters being combined, e. g. by addition, ratio formation or multiplication, to form a single wear condition parameter.
 7. Method according to claim 3, wherein the three-dimensional vibration signal is individually evaluated in each space axis, and, prior to transformation, combined by means of the wavelet function to form a resultant vibration signal, e. g. by addition, ratio formation, multiplication or vector addition, and the resultant vibration signal is transformed by means of the wavelet function to determine a wear condition parameter from the wavelet coefficients.
 8. Method according to claim 1, wherein the vibration signal is detected on the tool (2), the tool receiving means or the machine components directly or indirectly supporting the tool receiving means.
 9. Method according to claim 1, wherein the vibration signal is detected on the workpiece (4), the workpiece receiving means or the machine tool (1) components directly or indirectly supporting the workpiece receiving means.
 10. Method according to claim 1, wherein an acceleration sensor, a vibration velocity sensor, a vibration path sensor or, in the higher-frequency range, a solid-borne sound sensor or a sound emission sensor is used as a vibration sensor.
 11. Method according to claim 1, wherein the wear condition of a tool (2) is monitored during machining with interrupted cut.
 12. Method according to claim 11, wherein the wear condition of a milling tool is monitored.
 13. Method according to claim 12, wherein the wear condition monitoring of the milling tool is performed during free-form milling.
 14. Method for monitoring the wear condition of a tool (2) during machining of a metal workpiece (4) by means of a machine tool (1) by detecting measuring signals generated during the machining process with the aid of at least one sensor (6) and by transforming said measuring signals detected by said sensor (6) into wavelet coefficients by means of a wavelet function, the resultant wavelet coefficients being used for calculating wear condition parameters for the tool (2), wherein the vibrations, which are generated during the machining process, of a vibration system comprising at least the tool (2) and the metal workpiece (4) are detected by at least one vibration sensor (6), and in the case of repeated machining actions on similar workpieces (4) the wear condition of the tool (2) is evaluated on the basis of the change in the wear condition parameter relative to an initial parameter or relative to a given limit parameter at comparable cutting, engagement and/or contact conditions between tool (2) and workpiece (4) on similar or different workpieces (4).
 15. Method according to claim 1, wherein the given measuring locations with comparable cutting, engagement and/or contact conditions between tool (2) and workpiece (4) are determined from the data contained in the NC production program of the machine tool (1).
 16. Method according to claim 1, wherein the given measuring locations with comparable cutting, engagement and/or contact conditions between tool (2) and workpiece (4) are determined as a function of one or a plurality of the following parameters, namely workpiece blank geometry, workpiece finished part geometry, intermediate cut surfaces, machining parameters and tool geometry.
 17. Method according to claim 1, wherein, as a function of the wear condition parameters, the wear condition of the tool (2) is indicated in terms of quantity in a two-dimensional or three-dimensional representation of the tool along the cutting edge of the tool.
 18. Method according to claim 14, wherein, as a function of the wear condition parameters, the wear condition of the tool (2) is indicated in terms of quantity in a two-dimensional or three-dimensional representation of the tool along the cutting edge of the tool.
 19. Device for monitoring the wear condition of a tool (2) during machining of a metal workpiece (4) by means of a machine tool, the device comprising at least one sensor (6) for detecting measuring signals generated during the machining process, a signal evaluation means performing a wavelet transformation of the measuring signals detected by said sensor (6), and a computer for evaluating the wear condition as a function of the calculated wavelet coefficients, wherein said sensor (6) is a vibration sensor detecting the vibrations, which are generated during the machining process, of a vibration system comprising at least the tool (2) and the workpiece (4), and the computer evaluates the wear condition of the tool (2) on the basis of the change in the wear condition parameter at given measuring locations with comparable cutting, engagement and/or contact conditions on similar or different workpieces (4).
 20. Device according to claim 19, wherein the tool (2) consists a milling tool.
 21. Device according to claim 19, wherein the vibration sensor (6) consists of an acceleration sensor, a vibration velocity sensor, a vibration path sensor or, in the higher-frequency range, a solid-borne sound sensor or a sound-emission sensor.
 22. Device according to claim 19, wherein the vibration sensor is arranged on the tool, the tool receiving means or the machine components directly or indirectly supporting the tool receiving means.
 23. Device according to claim 19, wherein the vibration sensor is arranged on the workpiece, the workpiece receiving means or the machine tool components directly or indirectly supporting the workpiece receiving means.
 24. Device according to claim 19, wherein one vibration sensor is provided for each space axis.
 25. Device according to claim 19, wherein the vibration sensor is a three-dimensional sensor generating one vibration signal per space axis.
 26. Device according to claim 19, wherein a display unit is provided on which a two-dimensional or three-dimensional representation of the tool can be displayed, and the computer indicates, in terms of quantity and as a function of the wear condition parameters, the wear condition at the the worn locations on the representation of the tool.
 27. Device for monitoring the wear condition of a tool (2) during machining of a metal workpiece (4) by means of a machine tool (1), the device comprising at least one sensor (6) for detecting measuring signals generated during the machining process, a signal evaluation means performing a wavelet transformation of the measuring signals detected by said sensor (6), and a computer for evaluating the wear condition as a function of the calculated wavelet coefficients, wherein said sensor (6) is a vibration sensor detecting the vibrations, which are generated during the machining process, of a vibration system comprising at least the tool (2) and the workpiece (4), and the computer evaluates the wear condition of the tool (2) on the basis of the change in the wear condition parameter relative to an initial parameter or relative to a given limit parameter at comparable cutting, engagement and/or contact conditions between tool (2) and workpiece (4) on similar or different workpieces (4).
 28. Device according to claim 27, wherein the tool (2) consists of a milling tool.
 29. Device according to claim 27, wherein the vibration sensor (6) consists of an acceleration sensor, a vibration velocity sensor, a vibration path sensor or, in the higher-frequency range, a solid-borne sound sensor or a sound-emission sensor.
 30. Device according to claim 27, wherein the vibration sensor is arranged on the tool, the tool receiving means or the machine components directly or indirectly supporting the tool receiving means.
 31. Device according to claim 27, wherein the vibration sensor is arranged on the workpiece, the workpiece receiving means or the machine tool components directly or indirectly supporting the workpiece receiving means.
 32. Device according to claim 27, wherein one vibration sensor is provided for each space axis.
 33. Device according to claim 27, wherein the vibration sensor is a three-dimensional sensor generating one vibration signal per space axis.
 34. Device according to claim 27, wherein a display unit is provided on which a two-dimensional or three-dimensional representation of the tool can be displayed, and the computer indicates, in terms of quantity and as a function of the wear condition parameters, the wear condition at the the worn locations on the representation of the tool. 